Rework container creation

- simplify the process by having the riddler container build the rootfs and config
- output tarred up rootfs and config.json as otherwise file ownership not preserved
- allow easy build of a collection of container tarballs with another conversion script

This makes it easy to choose which container images you want and just convert any
set to a initrd image
```
tar cf - container1.tar container2.tar | docker run -i tartar2initrd > initrd.img
```

Next stage will use a manifest to select the ones to add for each edition.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

base/riddler/riddler.sh

@@ -2,6 +2,17 @@
 
 set -e
 
+# arguments are image name, prefix, then arguments passed to Docker
+# eg ./riddler.sh alpine:3.4 / --read-only alpine:3.4 ls
+# This script will output a tarball under prefix/ with rootfs and config.json
+
+IMAGE="$1"; shift
+PREFIX="$1"; shift
+
+cd /tmp
+mkdir -p /tmp/$PREFIX
+cd /tmp/$PREFIX
+
 # riddler always adds the apparmor options if this is not present
 EXTRA_OPTIONS="--security-opt apparmor=unconfined"
 
@@ -19,10 +30,25 @@ docker rm $CONTAINER > /dev/null
 # remove user namespaces
 # --read-only sets /dev ro
 # /sysfs ro unless privileged - cannot detect so will do if grant all caps
-# 
-cat config.json | \
+#
+mv config.json config.json.orig
+cat config.json.orig | \
   jq 'del(.process.rlimits)' | \
   jq 'del (.linux.resources.memory.swappiness)' | \
   jq 'del(.linux.uidMappings) | del(.linux.gidMappings) | .linux.namespaces = (.linux.namespaces|map(select(.type!="user")))' | \
   jq 'if .root.readonly==true then .mounts = (.mounts|map(if .destination=="/dev" then .options |= .+ ["ro"] else . end)) else . end' | \
-  jq '.mounts = if .process.capabilities | length != 38 then (.mounts|map(if .destination=="/sys" then .options |= .+ ["ro"] else . end)) else . end'
+  jq '.mounts = if .process.capabilities | length != 38 then (.mounts|map(if .destination=="/sys" then .options |= .+ ["ro"] else . end)) else . end' \
+  > config.json
+rm config.json.orig
+
+# extract rootfs
+EXCLUDE="--exclude .dockerenv --exclude Dockerfile \
+        --exclude dev/console --exclude dev/pts --exclude dev/shm \
+        --exclude etc/hostname --exclude etc/hosts --exclude etc/mtab --exclude etc/resolv.conf"
+mkdir -p rootfs
+CONTAINER="$(docker create $IMAGE /dev/null)"
+docker export "$CONTAINER" | tar -xf - -C rootfs $EXCLUDE
+docker rm "$CONTAINER" > /dev/null
+
+cd /tmp
+tar cf - .